RSNA: Long Needles Needed to Penetrate Fat in Obese

Action Points

Consider using a longer than standard needle when administering IM injections to overweight patients, particularly women.

This study was published as an abstract and presented orally at a conference. These data and conclusions should be considered to be preliminary as they have not yet been reviewed and published in a peer-reviewed publication.

CHICAGO, Nov. 28 - The growing epidemic of obesity means that many women and some men aren't getting the full benefit of vaccines and other intramuscular injections, researchers here said today.

The solution may be longer needles, said Victoria Chan, M.B., of the Adelaide and Meath Hospital in Dublin colleagues reported at the Radiological Society of North America meeting here.

Many medications designed to be delivered by injection into a major muscle group, such as the buttocks, don't make it into muscle when the buttocks are overly fat, said Dr. Chan. Indeed, in a prospective imaging study, Dr. Chan and colleagues found that more than two-thirds of injections failed to reach muscle tissue.

"Our study has demonstrated that a majority of people -- especially women -- are not getting the proper dosage from injections to the buttocks," Dr. Chan said. "There's no question that obesity is the underlying cause."

The implication of the study is that -- as the obesity epidemic expands -- more and more patients may not get the full benefit from intramuscular injections, she said. Because fat tissue has fewer blood vessels than muscle, it is less effective in distributing medications.

"This results in the patient either not receiving the maximum benefit of the drug or receiving no benefit at all, because the drug levels are insufficient to have any effect," Dr. Chan said.

The poorly aimed medication may also cause local infection or irritation in the fatty tissue, she said.

Dr. Chan and colleagues studied 50 patients, equally divided between men and women, who were at their institution for a CT scan of their abdomen and pelvis.

With their consent, the patients were given an intramuscular injection -- which included a small air bubble -- in the upper quadrant of the buttocks, using a standard 23-gauge needle.

The CT scans were analyzed by two independent radiologists to fix the position of the bubble, Dr. Chan said. The analysis found that:

Among the women, 23 of 25 injections -- or 92% -- failed to reach muscle.

Among men, 11 of 25 injections -- or 44% -- missed the mark.

Overall, the failure rate of the injections was 68%.

The take-home message for clinicians is that a longer needle may be needed, Dr. Chan said -- especially for women, who typically have more fat and less muscle in their buttocks than men.

"The more fat tissue there is in the buttock, the less likely the needle will reach the muscles underneath that fat," Dr. Chan said.

Reviewed by Zalman S. Agus, MD Emeritus Professor University of Pennsylvania School of Medicine